Since its invention 400 years ago, the astronomical telescope has evolved from a small, manually-pointed device for visual observations to a large and sophisticated computer-controlled instrument with full digital output. Throughout this development, two parameters have been particularly important. One is the light-collecting power or diameter of the telescope, which relates to the ability of the telescope to detect fainter and more distant objects. The other parameter is the angular resolution of the telescope, which relates to the image sharpness. For a perfect telescope used in a vacuum, resolution is directly proportional to the inverse of the telescope diameter. In such a telescope, a plane wave front from a distant star (i.e., effectively at infinity) would be converted into a perfectly spherical wave front, forming the image, with an angular resolution limited only by light diffraction-aptly called the “diffraction limit.”
In practice, both atmospheric errors and telescope errors distort the spherical wave front, creating phase errors in the image-forming ray paths. More particularly, turbulence in various layers of the atmosphere induces random spatial and temporal wave front perturbations. As a consequence, ground-based telescopes sited at even the best locations and observing at visible wavelengths cannot achieve an angular resolution better than telescopes of 10-centimeter (cm) to 20-cm diameter. For a 4-meter (m) telescope, atmospheric distortion degrades the spatial resolution by more than one order of magnitude compared with the diffraction limit. And the intensity at the center of the star image is lowered by a factor of 100 or more. In fact, one of the principal reasons for flying the Hubble Space Telescope was to avoid this image “smearing.” In addition, image quality is affected by permanent manufacturing errors and by long time scale wavefront aberrations introduced by mechanical, thermal, and optical effects in the telescope, such as defocusing, de-centering, or mirror deformations generated by their supporting devices.
Because it was thought that atmospheric distortions could not be avoided, development efforts were directed toward implementing mechanical improvements to minimize telescope errors. For example, mirror figuring and polishing techniques have been improved and stiffer structures and mirrors are now used to minimize gravitationally-induced deformations. Low-expansion glass was introduced to avoid mirror distortions with temperature variations. Additionally, heat dissipation from motors and electronic equipment was minimized during the night. In a properly designed and well-manufactured medium-size telescope, image quality is now limited mainly by atmospheric distortions.
By the mid 1980s, it became clear that conventional methods of maintaining image quality for telescopes with very large mirrors were not feasible due to cost and weight limitations. As a result, the technique of “adaptive optics” was developed for medium or large telescopes.
Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a spatial phase modulator, which is typically a deformable mirror. FIG. 1 depicts the conventional use of deformable mirror 100 to generate a corrected wavefront from an incoming distorted wavefront.
FIG. 2 depicts conventional adaptive optics system 200 for correcting for atmospheric turbulence. System 200 is shown in use with telescope 220. The adaptive optics system includes tilt mirror 202, deformable mirror 204, beam splitter 206, wavefront sensor 208, and processor/controller 210.
In operation, electrical signal DS that drives deformable mirror 204 is based on measurements obtained from wavefront sensor 208. More particularly, wavefront sensor 208 measures, in real time, the optical aberrations that remain after the corrections. Two methods are primarily used to measure the degraded wavefront. In one method, a “Shak-Hartmann” device is used. This device measures the slope of the wavefront from the positions of the images of the reference star, as given by each sub-pupil. The other method is “curvature sensing.” In this method, the intensities measured in strongly defocused images that are provided directly give the local curvatures of the wavefront.
For either method, wavefront sensing is typically performed on a reference or “guide” star. The observed object itself can be used for wavefront sensing if the object is bright enough and has sufficiently sharp light gradients. If not, an artificial guide star—a laser—is used. The measurement can be performed in the visible range for observation in the infrared, or in the infrared range itself if, for example, the reference star is too faint in the visible range.
Wavefront sensor 208 sends electrical signal SS, which contains wavefront measurement information, to processor/controller 210. The processor/controller processes the measurement information and determines, based on this information, how to alter the shape of deformable mirror 204 to achieve near-zero optical aberration. Processor/controller 210 then generates electrical drive signal DS, which is received by deformable mirror 204. The shape of mirror 204 is altered, based on the drive signal (discussed further below). A servo system or feedback loop is thus created (206→208→210→204→206) to obtain near-zero aberration by continuously adjusting the shape of deformable mirror 204. The light that passes through beam splitter 206 is optically processed (e.g., focused, etc.) by optics 212 to create the final image 214.
It will be appreciated that the correction process must be performed very quickly (within about 0.5 to 1 ms), otherwise the state of the atmosphere may have changed, rendering the wavefront correction inaccurate. (The required computing power can exceed several hundred million operations for each set of commands sent to a 250-actuator deformable mirror.)
Because of the high bandwidth and the small field to which correction can generally be applied, adaptive optics uses a small deformable mirror with a diameter of 8 to 20 cm located behind the focus of the telescope, at or near an image of the pupil. In some current projects, the possibility of using a large deformable secondary mirror is being developed. The choice of the number of (usually piezoelectric) actuators is a tradeoff between degree of correction, use of faint reference sources, and available budget. For instance, a near-perfect correction for an observation done in visible light with an 8-m telescope would require about 6400 actuators, whereas similar performance in the near infrared range requires only about 250 actuators.
A large number of actuators require a similarly large number of sub-apertures in the wavefront sensor. This means that for correction in the visible range, the reference star should be about (6400/250) or 25 times brighter than if correcting in the infrared range. Most current astronomical systems are designed to provide diffraction-limited images in the near-infrared range with the capability for partial correction in the visible range. Some military systems for satellite observations in the USA do, however, provide full correction in the visible range on at least 1-meter class telescopes.
The deformable mirror is controlled using zonal or modal control methods. In zonal control, each zone or segment of the mirror is controlled independently by wavefront signals that are measured for the sub-aperture corresponding to that zone. In modal control, the wave front is expressed as the linear combination of modes that best fit the atmospheric perturbations.
There are several types of deformable mirrors that can be used in adaptive optics systems. One type is a segmented deformable mirror, an example of which is depicted in FIGS. 3A and 3B. Segmented deformable mirror 322 comprises an array of independently actuated flat mirror segments 324. Each segment 324 can move a small distance back and forward, based on the operation of piston-type actuators 326A and 326B, to approximate the average value of the wave front over the area of the segment. These type of mirrors typically exhibit little or zero cross-talk between actuators.
But such stepwise approximation of the wavefront works poorly for smooth continuous wave fronts. In particular, sharp edges of the segments and gaps between the segments contribute to the light scattering, thereby limiting the applications to those that are non-sensitive to scattered light. Considerable improvement of the approximation performance of the segmented mirror can be achieved by introducing three degrees of freedom per segment: piston, tip, and tilt. These mirrors require three times more actuators than piston-segmented mirrors and they suffer from diffraction on the segment edges.
Another type of deformable mirror is the continuous faceplate deformable mirror, as typified by mirror 422 of FIGS. 4A and 4B. This type of deformable mirror includes a plurality of discrete actuators 426 that abut the back surface of thin deformable reflective faceplate 424. The shape of faceplate 424 is controlled by the operation of the discrete actuators.
Some other embodiments (not shown) of the continuous faceplate deformable mirror have discrete actuators positioned on the backside of a deformable plate, the front side of which receives a reflective face plate. The discrete actuators alter the shape of the plate, which in turn, alters the shape of the face plate. In any case, the continuous faceplate deformable mirror is considered to be among the best of the deformable mirrors because they enable smooth wavefront control with a very large number of degrees of freedom.
Yet another deformable mirror is the membrane deformable mirror, which is formed by a thin conductive and reflective membrane that is stretched over a solid flat frame. The membrane can be deformed electrostatically by applying control voltages to electrostatic electrode actuators that are positioned under or over the membrane. If there are any electrodes positioned over the membrane, they are transparent. It is possible to operate the mirror with only one group of electrodes positioned under the mirror. In this case, a bias voltage is applied to all electrodes, to make the membrane initially spherical. The membrane can move back and forth with respect to the reference sphere.
A further type of deformable mirror is the bimorph deformable mirror. This type of deformable mirror is formed by two or more layers of different materials. One or more (active) layers are fabricated from a piezoelectric or electro-strictive material. An electrode structure is patterned on the active layer to facilitate local response. The mirror is deformed when a voltage is applied to one or more of its electrodes, causing them to extend laterally, which results in local mirror curvature. Bimorph mirrors are rarely made with more than 100 electrodes.
MEMS-based deformable mirrors have been fabricated using bulk or surface micromachining. An advantage of MEMS mirrors is that they can be inexpensive compared to other deformable mirrors.
To correct turbulence for extremely large telescopes (30 to 100 meters in diameter) in the visible range, deformable mirrors with 10,000 to 100,000 actuators will be required. One approach for producing these mirrors relies on MEMS processing, using micro or nano-lithographic techniques. The resulting small mirror elements would be deflected by electrostatic forces. The problem with this type of approach is the insufficient stroke of the actuation system, not to mention the exceedingly large number of actuators that are required.
As a consequence, there is a need for a different approach to actuating a deformable mirror for use in an adaptive optics system.